Thermal ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink that entered from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of print-head and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected through a conductive trace to a microprocessor where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber which is expelled through the nozzle toward the print medium. In this way, firing a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fills, and provides other print capabilities on the medium.
Ink-jet inks used in thermal ink-jet printing typically comprise a colorant and a vehicle, with the vehicle often containing water and other relatively low surface tension liquids.
The tight tolerances of the nozzles, typically 50 .mu.m diameter, require that the ink not clog the nozzles. Further, repeated firings of the resistor elements, which must withstand about 10 million firings over the life of the ink cartridge, can result in fouling of the resistor element. Finally, the ink composition must be capable of inter-acting with the print medium, especially paper, to penetrate the paper without undue spreading, and the ink composition should be smear- and water-resistant on the paper.
Inks are known which possess one or more of the foregoing properties. However, few ink compositions are known that posses all of those properties, since an improvement in one property often results in the degradation of another. Thus, inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate performance in each of the aforementioned properties.
In a related application, application Ser. No. 08/794,097, filed Oct. 31, 1996, a new class of colorants for ink-jet inks, namely, macromolecular chromophores (MMCs) are defined. These materials are commercially available from colorant vendors, such as Cabot Corporation and Orient Chemical. As disclosed therein, the macromolecular chromophores comprise pigments which have been chemically modified to impart water solubility to the otherwise water-insoluble pigments by introducing carboxylate and/or sulfonate functionalities onto the pigment surface (to generate anionic chromophores) or by introducing ammonium or phosphonium functionalities onto the pigment surface (to generate cationic chromophores).
In another related application, application Ser. No. 08/742,789, filed Oct. 31, 1996, the chemistries involved for exchanging counterions associated with the functional groups on the MMCs to impart improved properties to the inks using these MMCs. Any of the common ion exchange processes may be used to exchange one counterion for another.
Investigations continue into developing ink formulations that have improved properties such as high edge acuity, high optical density, fast drying times, good waterfastness, and good smearfastness without sacrificing performance in other necessary properties.